1. Field of the Invention
This invention relates to adjustment of the protection curves of electric circuit protection devices such as circuit breakers and overload relays, and for coordinating the protection curves of a hierarchy of such protection devices.
2. Background Information
Protection devices such as circuit breakers and overload relays in electric power distribution systems interrupt the current if certain current/time characteristics are exceeded. Specifically these protection devices have a rated current which can be maintained indefinitely. Very high currents, many times in excess of the rated current of the device, such as those associated with a short circuit, result in an instantaneous trip. Some loads, such as electric motors during start-up, draw currents in excess of rated current for a period of time. Such temporary overcurrents must be tolerated if such loads are present. However, if the motor is stalled or the overcurrent otherwise persists, the current must be interrupted to protect the load and the power conductor from overheating. The long delay trip function permits such temporary overcurrent conditions. If the current exceeds a long delay pick-up value, a long delay timing function is initiated. The time to the delayed trip is usually associated with the type of load, for instance the period of overcurrent expected for the class of the electric motor. Typically, the trip curve is set for a trip to occur if the current remains at or above a specified factor, such as 3 to 6 for instance, times the rated current for the long delay time period. However, if the current remains above the long delay pick-up value but below the selected factor times the rate of current, a trip will occur at a time which is an inverse function of the magnitude of the overcurrent. Various inverse time functions can be used, but most commonly an I.sup.2 t function (where I is the current and t is the time in seconds) is used.
Typically, projection devices are provided at several levels in the electric power distribution system. Thus, a main circuit breaker is used in the main bus and additional circuit breakers are provided in the feeder circuits and typically in the branch circuits also, the later of which may further include an overload relay and a starter for a motor. This creates a hierarchy of protection devices with the main circuit breaker at the top and the branch circuit breakers or the overload relays at the bottom. It is desirable to have the lowest circuit breaker possible in the hierarchy respond to a fault or overcurrent condition to minimize disruption of service. This requires coordination of the operation of the protection devices which is implemented by a short delay trip function. The short delay trip function allows a circuit breaker to tolerate an overcurrent of a magnitude which is greater than the long delay pick-up value but less than the instantaneous trip value for a short interval of time in order to allow a lower circuit breaker in the hierarchy to respond.
Coordination of the trip curves of the circuit breakers in an electric power distribution system has traditionally been implemented by drawing the protection curves for the circuit breakers on separate transparencies which are overlaid. If satisfactory margins between the overlaid curves do not exist, the parameters of the affected circuit breakers are adjusted and replotted until satisfactory margins are realized. This coordination process requires a highly skilled engineer. The various parameter values for each of the circuit breakers are given to a technician who enters them into the respective devices. This process is time consuming, costly, and subject to human error.
It is known to display individual trip curves on a computer monitor where the curve can be adjusted by textual entries of the various parameters. Separately, it is known to download circuit breaker parameter values from a remote computer using network communications. This potentially reduces the human errors associated with inserting the correct parameter values in the correct circuit breaker. However, there is no comprehensive system or process for improving the integrity and reducing the required time and cost of coordinating the trip curves of protection devices in an electric power distribution system.
Hence, there remains a need for improved apparatus and a method for coordinating the trip curves of the protection devices in an electric power distribution system.
More particularly, there is a need for such an improved method and apparatus which improves the integrity of the coordination of the protection devices while reducing the cost, time and number and skill of personnel required.
There is also a need for improved apparatus and a method for adjusting the trip curve of an individual circuit breaker.